Discontinued Operations and Assets Held for Sale (Table Illustration of Number of Properties in Discontinued Operations) (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2013 buildings	Dec. 31, 2012 buildings
Number of properties held for sale in discontinued operations	2
Number of properties sold in discontinued operations	6	28
Total number of properties in discontinued operations	36
Office [Member]
Number of properties held for sale in discontinued operations	0
Number of properties sold in discontinued operations	1	10
Total number of properties in discontinued operations	11
Industrial [Member]
Number of properties held for sale in discontinued operations	1
Number of properties sold in discontinued operations	3	17
Total number of properties in discontinued operations	21
Medical Office [Member]
Number of properties held for sale in discontinued operations	0
Number of properties sold in discontinued operations	2	0
Total number of properties in discontinued operations	2
Retail [Member]
Number of properties held for sale in discontinued operations	1
Number of properties sold in discontinued operations	0	1
Total number of properties in discontinued operations	2